In traditional machine designs several sliders are stacked to generate 2- and 3-axis machines. With conventional use of scales, it is not possible to measure directly the position of a slide, which moves also perpendicular to the measurement direction. As a result the measurement system often consist of a simple stacking of several linear scales together with the supporting slides. In this way the position of the end effector is not directly measured and Abbe offsets are present. Although high quality scales are used, the final performance will depend on the straightness and orthogonality of the different guide ways. Often the system is calibrated to correct for these errors, but still deformations due to thermal effects and due to the flexibility of the bearings and the structure will reduce the final performance. FIG. 1 gives an example where the X- and Y-position of the slide 1 is respectively derived from the X-scale 2 and Y-scale 3.
A combination of a traditional use of scales for long stroke measurements, with short stroke sensors to measure straightness deviation and deformations could be used. This however requires a high number of sensors and reference straight edges. Plane laser interferometry is traditionally used to overcome this problem. However, while the sensitivity for environmental changes of scales is limited to the expansion of the scale, laser interferometry on the other hand is influenced by pressure, humidity, temperature and air composition.
In laser interferometry efforts are focussed on maximizing the stability of the measurement. Laser beams in vacuum or He— atmosphere offer the ultimate performance, but are also expensive. Common practice is to correct for the environmental disturbances by using tracking refractometers or by measuring pressure, temperature and humidity. The final result will still be dependent on the homogeneity of the air and the presence of turbulences. Especially for machine tools where measurements should be done during motion and cutting fluids are present, it is difficult to quantify these disturbances. Shielding the laser beam or creating a more homogeneous environment by introducing airflow is often applied, but this is also more difficult to accomplish for multi-axis machine tools compared to for instance single axis measurement machines.
In the field of scales, a special configuration based on scales offering direct and Abbe free measurement exists (Van Seggelen, J. K. et al. (2002), ‘Design of a 3D CMM with elastically guided z-axis and x, y axis with less than 2 mm ABBE offset’, Proc. of the 3rd euspen International Conference 1, 29-32). In the cited configuration the scales for the X- and Y-measurement are moving to obtain an Abbe free measurement in X- and Y-direction. This configuration involves however two sliders for each moving scale of which the straightness of one of the sliders directly influences the measurement accuracy. Extending this solution for a target translating in three directions is also difficult/impossible. The more recent 2D-grating scales are of course also limited to 2D-translations.